The present invention relates to an improvement of a liquid-crystal projector.
A liquid-crystal projector projects an image on a transmission type liquid-crystal display panel, and the image displayed on the liquid-crystal panel is enlarged and projected onto a screen surface. The liquid-crystal projector has the following arrangement.
FIG. 1 shows the schematic arrangement of a conventional liquid-crystal projector. Reference numeral 1 denotes a transmission type dot-matrix liquid-crystal panel; and 2, a light source lamp arranged behind panel 1. Light emitted from lamp 2 is reflected by reflector 3 and illuminates panel 1 from behind through condenser lens 4 and relay lens 5. Reference numeral 6 denotes a focusing lens arranged in front of panel 1; and 7, a projection lens. Light passing through panel 1, i.e., light beams corresponding to an image displayed on panel 1, is focused on projection lens 7 by focusing lens 6, and is enlarged thereby to be projected on the surface of screen S.
The liquid-crystal projector allows watching of an enlarged image displayed on a liquid-crystal display panel (e.g., a television image). With this projector, an image displayed on a liquid-crystal display panel having a small screen can be watched on a large screen.
However, since the liquid-crystal projector enlarges a displayed image on panel 1 using projection lens 7 and projects it onto the surface of screen S, the outer image portion projected on screen S has a lower brightness than that of the central image portion.
This is caused by a difference between optical path lengths from the respective portions of panel 1 to screen S. Since outer light beams propagating toward screen S through lens 7 have a longer optical path length than that of inner beams, the luminance of the outer light beams is greatly attenuated, thus darkening the outer image portion projected on screen S. This phenomenon is common in normal projectors which enlarge and project a film image onto a screen surface. However, in the normal projector, since the luminance of light is only slightly attenuated when it passes through a film, the above-mentioned phenomenon is not noticeable. In contrast to this, in the liquid-crystal projector, since light attenuation by the liquid-crystal display panel influences the brightness of the image projected on the screen, the above phenomenon is noticeable.
FIG. 2 shows the brightness of image A projected on the surface of screen S by the conventional liquid-crystal projector. Upper and lower portions Aa and Ab of image A are darker than the central portion thereof. Widths Wa and Wb of portions Aa and Ab each correspond to 1/4 of total width W0 of image A. Note that FIG. 2 exemplifies a state wherein only upper and lower portions Aa and Ab of image A are darkened. However, such a phenomenon also occurs on the right and left side portions of the screen.